Implement `BindingMode` for pattern matching, so that types can be
correctly inferred using match ergonomics. The binding mode defaults to
`Move` (referred to as 'BindingMode::BindByValue` in rustc), and is
updated by automatic dereferencing of the value being matched.
It was just getting too big. We now have:
- ty: the `Ty` enum and helpers
- ty::infer: actual type inference
- ty::lower: lowering from HIR to `Ty`
- ty::op: helpers for binary operations, currently